Xylene isomerization with hydrogenation



March 5, 1957 M. M. HOLM XYLENE ISOMERIZATIQN WITH HYDROGENATION 2 Sheets-Sheet 1 Filed June 6, 1951 TOTAL PRESSURE ATMOSPHERE INVENTOR MELV/N M. HOLM MM ATTOREYS Unite XYLENE ISOMERIZATION WITH HYDROGENATION Melvin M. Helm, San Francisco, Calif., assignor to California Research Corporation, San Francisco, Calif., a corporation of Delaware Application June 6, 1951, Serial No. 230,168

8 Claims. (Cl. 260-668) States Patent non-equilibrium mixture of xylene isomers may be converted to a substantially equilibrium mixture.

Recently, efficient methods have been developed for separating individual xylene isomers from xylene mixtures and commercial uses for xylenes have been developed in which individual isomers of high purity are required. Levine Patent No. 2,474,002 describes a-sep-- aration of ortho-xylene from mixed xylene isomers and the, conversion of ortho-xylene to phthalic anhydride. Arnold Patent No. 2,541,682 describes jan'eificient'nietliod for a separating para-xylene from a mixture of xylene isomers" by fractional crystallization. 'Para-xylene, after separation, is used as a starting material in the production of. Brooke et al. Patent No. 2,521,444

terelin-type fibers. describes a method for separating meta-xylene from mixed xylene isomers by extracting xylene mixtures with HF-BFa.

In an equilibrium mixture of xylene isomers, the in process for isomerizing individual Xylene isomers and non-equilibrium mixtures of xylene isomers.

It has now been found that individual xylene isomers and non-equilibrium mixtures of xylene isomers may be isomerized to produce substantially equilibrium xylene mixtures by contacting the individual xylene isomer or the non-equilibrium mixture and hydrogen gas with a hydrogenatioil-dehydrogenation catalyst under hydrogenating conditions toconvert a substantial proportion or all of the xylenes to naphthenes and then contacting the hydrogenated material and hydrogen with a hydrogenation-dehydrogenation catalyst under dehydrogenating conditions to reconvert the naphthenes to xylenes. In the course of this reconversion'a substantially equilibrium mixture of xylenes is formed.

In one embodiment of the invention a two-stage process is conducted. In the first stage xylenes and hydrogen gas are contacted with a hydrogenation-dehydrogenation catalyst under hydrogenating conditions to convert a substantial proportion of the xylenes to naphthenes. In the second stage the naphthenes produced in the first stage are contacted with a hydrogenation-dehydrogenation catalyst under dehydrogenating conditions to reconvert the naphthenes to xylenes.

The terms hydrogenating conditions, dehydrogenation conditions and "hydrogenation-dehydrogenation catalysts are generally well understood in the art.

The term hydrogenating conditions indicates the employment of relatively low temperatures and relatively high hydrogen pressures in the operation. The temperatures employed are ordinarily below about 800 F. and

dividual isomers are present in approximately the following relative amounts:

Percent I Ortho-xylene Q. 1 13 to 28 Meta-xylene 641050 f Para-xylene 23-to 22 One commercial methodfor producing mixed xylene isomers consists in hydroforming distillationfractions of naphthenic crude oils. The hydrofor-mate is fractionally distilled to separate a C8 aromatic hydrocarbon fraction which contains ethylbenzene in addition to the threeisomers, a residual xylene fraction containing the 1m desired isomers is recovered as a raflinate, a motherliquor, or a distillation fraction, depending upon the particular method of separation employed. This residual fraction ordinarily contains more than half of'the total xylene material which is subjected to the separation process. In such an operation it would be highly desirable to isomerize the residual xylene fraction to produce further quantities of the desired isomer which could be recovered for the intended use. Numerous proposals have been made for isomerizing xylenes, but no one of the proposals heretofore made has been successful in the sense of being sufficiently eflicient to be reduced to commercial practice. xylene isomerization process is in operation.

It is an object of this invention to provide anelficient At the present time no commercial about 25 atmospheres.

usually in the range about 400 to 800 F. The hydrogen pressures employed range from a minimum level of about 20atmospheres up to 200 atmospheres and higher. Hydrogenation occurs at the lower pressures in this range,

provided that lower temperatures Within the temperature range are employed, while higher pressures must be employed if higher temperatures within the temperature range are used. This relation between temperature and hydrogen pressure is well understood by those skilled in the art.

The term dehydrogenating conditions is generally understood to include relatively high temperatures and relatively low hydrogen pressures in the operation. Temperatures above 800 F. and up to 1200 F. may be employed and hydrogen pressures are ordinarily below The use of hydrogen pressure ina dehydrogenation operation serves the purpose of reducing the rate at which coke is deposited on the catalyst. When' relatively 'high teniperatures are employed in the dehydrogenation process, for example, temperatu'res at about 1000 F., it is possible to employ fairly'high. pressures of hydrogen in the process for the purpose of. prolonging catalyst life. This type of operation has come into commercial practice and is generally described as catalytic reforming or, hydroforming.

Hydrogenation-dehydrogenation catalysts broadly include the metals and the oxides and sulfidesof the metals to 800 F. in the portion ofthe bed adjacent to the inlet for xylenes and hydrogen, to a substantially higher temperature usually in the range 800 to 1000 F. in the portion of the catalyst bed adjacent to the product outlet. In this mode of operation the conditions in the portion of the bed adjacent to the inlet are hydrogenating conditions and the conditions in the portion of the catalyst bed adajcent to the product outlet are dehydrogenating conditions. The xylenes in passing through the catalyst bed in which a gradient temperature of this character is maintained are first hydrogenated to naphthenes in the low temperature portion of the bed and then dehydrogenated to produce an equilibrium mixture of xylenes in the high temperature portion of the bed.

In another embodiment of the invention xylenes and hydrogen are contacted with a hydrogenation-dehydrogenation catalyst under conditions adapted to produce an molybdena-alumina catalyst described in Claussen Patent No. 2,432,286 and now used in practically all commercial hydroforming operations was employed. Run 1315 is a normal hydroforming operation in which a straight-run naphthenic distillate boiling from 180 to 325 F. is charged to the operation. In run 1374 the feed is the mother-liquor recovered after crystallization of para-xylene from a xylene-rich fraction separated from catalytically reformed naphtha which contained about 17% para-xylene, 47% meta-Xylene, 10% ortho-xylene, 12% paratfins and 12% ethylbenzene. The conditions in this run were essentially the same as those in run 1315. In run 1375 the feed is again the mother-liquor from a paraxylene crystallization process, but contact with the catalyst is made in the presence of methane instead of hydrogen.

In the runs summarized in Table I the distribution of the aromatics is shifted generally in the direction of equi- TABLE I Xylene zsomerzzatzon and recycle m the catalytic reformer Run No 1315 1374 1375 F St. Run 5 Mother- Mother- Liquor 1 Liquor (Ha atm.) (CH atm.) Run Conditions: 2

Temp, F. eacter Inlet) 955 965 965 Space Rate, .[V./hr 1 1 1 Pressure, p. s. 1 g 200 200 200 Recycle aromatics in feed:

HTI7AYIQ Toluene Ethyl Benzen 16. 5 18. 4 1! o-xyl m 4. 4 4. 9 m-xylen 58. 4 65. 1 p-xylene- 1D. 4 11. 6

89. 7 Aromatics in Product: 3

2. 5 3. 2 .6. 3 2. 6 14. 6 14. 0 All Aromatics 43. 2

1 Mother-liquor from p-xylene crystallization.

I Adrabatic reactor; molybdene-alumina catalyst; gas recycled at 6,000 cu. ft./bbl. of feed. I Vol. percent of total teed. I

Vol. percent 01' raw feed (straight run only).

. straight run naphtha t Blocked numbers are 0; aromatic distribution.

hydrogenation begins to occur and a dynamic equilibrium u in which xylenes are being hydrogenated to naphthenes and naphthenes are being dehydrogenated to xylenes is established. In this mode of operation the catalyst is maintained at a temperature in the range of 700 to 900 F. and the partial pressure of hydrogen is varied with the temperature to achieve the desired equilibrium condition from a value of 12 to 18 atmospheresat 700 -F. to 60 to 96 atmospheres at 900 F. The space velocity employed in this operation is in the range 0.1 to 3.0.

Since the hydroforming of straight-run naphthenic dis L V tillates of crude oils produces a substantially equilibrium mixture of xylene isomers, it was thought that recycle of a residual xylene fraction to the hydroformingi unit after the separation of an individual isomer might etlect isomerization of the residual fractionrtofl produce: an

equilibrium mixture. Attempts to, isomerize xylenes. or increase the production of a desired isomer infthismanner were. unsuccessful as shown in the data in the following Table 1n -the. of Table I the coprecipitated ing ordemethylation.

librium with respect to the isomers present, but there is no net gain in the production of para-xylene. So far as the data including liquid product yield showed, there may have been no isomerization of .xylenes, but merely a pre ferential-destruction of orthoand meta-xylenes by crack- In the following Example 1, the isomerization of xylenes by a two-stage process'in which the xylenes are hydrogenated in the first stage and the produced naphthenes are dehydrogenated in the second stage to produce an approximate equilibrium mixture of xylene isomers is illustrated, 1

p 7 "nxAMrLn A sample of pure meta-xylene was hydrogenated over Raney nickelcatalyst under conditions that yielded a single naphthene isomer that was shown by spectrometric analysis to be 99+% as 1,3 dimethyl cyclohexane. This material was then dehydrogenated over a molybdenaalumina hydrotorming catalyst under the following conditions: V

Pressure 200 p..s. i. g.

Reactor inlet temperatureu n'- 95511 Liquid space rate 1 v./v./hr. Gas recycle 6000 cu.ft./bbl. of feed.

The yields (vol. percent) of aromatics produced were as follows:

The following example illustrates the embodiment of the invention in which a temperature gradient is maintained in the catalyst bed.

EXAMPLE 2 The mother-1iquor recovered from a fractional crystallization process for the separation of para-xylene from a substantially equilibrium mixture of xylenes produced Isomerlzatioa of eylene mother-liquor0ontinued Feed Product Liquid:

Gravity, A. P. I 32. 7 48.1 44. 8 Aniline Point, F 31 1 Fractionation:

Start, 194 F 21. 7 23. 2 19 248 23. 6 22.0 248-300 96 4 50. 5 48. 9 300+Btms 3 6 4.2 5. 9 Analysis 01248 to 300 Gut:

Ethyl Benzene 10.6 6. 8 7. 2 15. 1 16. 2 21. O 62. 3 35. 8 39. 3 4. 9 17.1 18. 3

NOTES:

These runs were made in a non-isothermal, non-adiabatic unit with catalyst in 2 inch I. D. chamber surrounded by heated bronze blocks. Catalyst depth was 10 inches.

Catalyst was i0" molybdena-alumina pellets having a molybdeno content of 9%.

The following example illustrates the embodiment of by catalytic reforming was isomerized by passing the the invention in which a non-equilibrium mixture of xylene vapors and hydrogen gas through a bed of hydroxylene isomers is contacted with a hydroge11ation-dehygenation-dehydrogenation catalyst. A temperature gradidrogenation catalyst under conditions adapted to produce ent ranging from a lower temperature at the inlet end an equilibrium mixture of xylenes and naphthenes conof the bed to a substantially higher temperature at the taining 10 to 30 mole percent of naphthenes. outlet end of the bed was maintained. The reaction at the EXAMPLE 3 1 Inlet end the bed Y Pnmanly hydrogenatlon of The mother-liquor recovered from afractionalcrystalli- Wh 11e the reactlon the outlet end of the bed zation process in which para-xylene was separated from was P dehydrogenatlon of naphthenes to Produce a substantially equilibrium mixture of xylenes produced an equ1l1br1um m xture of xylenes. The data for two run by catalytic reforming was isomerized by passing the under these conditions are summanzed 1n the following Xylene vapors and hydrogen through a bed f hydrogena Tab 1 non-dehydrogenation catalyst under conditions of tem- TABLE II perature and hydrogen pressure adapted to produce an equilibrium mixture of xylenes and naphthenes having lsomerzzatzon of xylene mother-liquor a naphthene content in the range about 10% to about 30%. In run 4-.1173 the coprecipitated molybdena- Run No 4 1164 alumlna catalyst of Claussen Patent No. 2,432,286 was employed. In runs 4-1214, 4-1215 and in run 4-1232 gressugatp. s./i.g/1 1 302 510 the catalyst was platinum on alumina. The platinum ace a e, v. v. r Rate, Mole/Mole Feed -33 -25 catalyst used 1n runs 4-1214 and 4-1215 had the follow- TemTperagillrelsifiR: 775 790 mg composmon: 1% platlnum on a pure precipitated gp 3 156k?- 825 825 alurnlna, and was prepared by impregnation with chlor- Catalytst topd 797 785 platlmc ac1d followed by calcmatlon at 1100 F. That lnafyrge 850 835 used in run 4 1 232 was a commercial platinum-alumina Intermediatepointincat- 900 900 catalyst containlng approximately 0.3% platinum. Z FE 885 910 5 Runs under these conditions are summarized in the followmg Table III. TABLE III Isomerizatton of mylene mother-liquor Run No 4-11730 4-1214 4-1215 4-1232 Pressure p. s. i. g..-.-.. 800 800 800 S ace Rate v.{v./hr. 3 1 1 7 Rate m0 e/mol of v F d 15 6 6 3 900 762 710 C .1 T a; a2 2 iii a ys op. 53 872-875 860 905 855 Intermetgiate; 111011111; 935 872 875 inca ys e 868 872- Bottom 900 30 872-272 Yields: Liqui Vol.

Percent 8. 5 9 1 101. 4 5

Feed Feed 1st Hr. 2ndHr. Feed Feed Hrs.1and2 Hrs.3and4 Liquid:

Gravity, A. P. 1.-.. 34.0 35.8 (4-11730.) 46.4 43.2 (4-11730.) 43.9 34.0 .0 35.1 Aniline Point, F.- 1 3 27 44 4 Fractionation:

s 1.1 10.0 14.7 817.195 0 4.3 1.8 194-248-.-- 9.1 -22.0 21.0 195-240 0 6.1 5.2 248-300 98.4 87.0 53.4 58-4 240-270} 4 4.8 300+Bottorns 1.6 2.8 5.6 5.9 270-300 79.2 82.9 Btms., 1.6% 5.6 5.2 Algjalysis of 248 to 300 11 I Ethyl Benzene 19.7 14.4 17.1 10.0 11.6 10.5 11.2 10.1 19.7 15.8 1. O-Xylene 3- 10.7 12.5 14.1 15.1 1&7 7.0 m8 m-xylene 43.5 51.7 46.3 53.9 47.6 51.1 50.8 47.8 51.2 53.0 49.8 p-xylene 14.7 17.5 1 22- 2L1 22- 22-4 11.8 16,7 17,3 1&6 Tom .3 4,1 85.9 93.3 86.3 96.5 94.7

Figure l of the appended drawings is a graphical repre-- sentation of the distribution of xylenes and naphthenes at equilibrium under varying conditions of temperature and pressure. From the graph it will be seen that an equilibrium mixture containing from 10 to 30 mole percent of naphthenes can be produced by contacting xylene vapors and hydrogen with a hydrogenation-dehydrogenation catalyst at temperatures from 700 to 900 F. and hydrogen partial pressures ranging from approximately 12 to 18 atmospheres at 700 F. to 60 to over 100 atmospheres at 900 F. When non-equilibrium mixtures of xylene isomers are subjected to these conditions, the xylenes are isomerized to produce substantially equilibrium mixtures.

Figure 2 of the appended drawings diagrammatically illustrates a suitable process flow for use in isomerizing xylenes pursuant to the invention. The xylene feed and hydrogen are introduced through line 10 into catalytic reactor 1, where they contact a hydrogenation-dehydrogenation catalyst, for example, platinum on alumina. 0 The reaction mixture produced during this contact passes through line 15 and heat exchanger 6 into catalytic reaction zone 2, where it contacts another body of thehydrogenationdehydrogenation catalyst. The reaction product mixture produced in reactor 2 flows through line 16 and heat exchanger 7 into catalytic reaction zone 3, where it contacts still another body of the hydrogenation dehydrogenation catalyst. Catalytic reactors 1, 2 and 3 are operated under conditions such that a substantial proportion of the xylene is hydrogenated to form naphthenes. For example, these reactors are maintained at temperatures within the range 600 to 800 FQand at a pressure about 800 p. s. i. g., the partial pressure of hydrogen in the reactors being about 500 to 600 p. s. i. g. r The xylene feed is contacted with each of the three catalyst masses at space velocities in the range about 0.1 to 2.0 liquid v./v./hr. The hydrogenation reaction occurring in reactors 1, 2 and 3 is highly exothermic. Interstage cooling between the successive reaction zones is provided. The efiiuent from reaction zone 1 is cooled by passing through heat exchanger 6 and is further cooled by the introduction of cold feed and hydrogen into line 15 through line 11. Similarly, the effluent from reaction zone 2 is cooled by passing through heat exchanger 7 and further cooled by the introduction of additional cold feed and hydrogen into line 16 through line 12.: The eifiuent from reaction zone 3 passes through pressure reduction valve 14 and furnace 8 into reaction. zone 4, where it is contacted with an additional body of the hydrogenation-dehydrogenation catalyst. The efliuent 59 from reaction zone 4 passes through" line 18 and furnace 9 into reaction zone 5, where it is contacted with still another body of the hydrogenation-dehydrogenation catalyst. The reaction product comprising isomerized xylenes is withdrawn from reaction zone 5 through line 5 13. Reaction zones 4 and 5 are operated under dehydrogenating conditions to convert the naphthenes formed in reaction zones 1, Z and 3 to xylenes. Suitable conditions for employment in reaction zones 4 and 5 are temperatures in the range 950 to l050 F., -and a total pres- 69 sure about 600 p. s. i. g., the partial pressure of.hydrogen. being in the range 300 to 400 p. s. i. g. The space velocity in reaction zones 4 and5 is higher than thatemployed in reaction zones 1, 2 and 3, being from 1.2 to about 1.8 times the space velocity employed in the latter 1 reaction zones. The dehydrogenation reaction occurring in reaction zones 4 and 5 ishighly endothermic.- Accordingly, the effluent from reaction zone'S "is passed through pressure reduction valve 14 to reduce the pressure by l00 to 300 p. s. i. This pressure reduction pro- 70 duces an increase in the temperature of the effluent from reaction zone 3. The temperature of the efiluent from reaction Zone 3 is further increased by passing the etiluerit' passingit through furnace 9 prior'to its introduction into reaction zone 5. i

The flow Patinaass r ndi i as.i stra e scribed with reference to Figure Znray be utilized iu isomerizing a single xylene isomer, or a non eq uilibrium mixture of xylene isomers. Also; a mixture of xylenes and Ca naphthenic hydrocarbons ma be employed as. the feed to the process. Up to about of the total. feed may be naphthenic. By using a mixture of xylenes and naphthenes, temperature control is more easily maintained in reaction zones 1, 2 and 3 and the overall process accomplishes net production of xylenes in addition toxylene isomerization. P a

I claim:

1. A process for recovering a desired xylene isomer from a mixture of xylene isomers having substantially less than the equilibrium content, of the desired isomen which comprises contacting said xylene mixture in vapor,

phase and hydrogen-with a hydrogenation-dehydrogenation catalyst under conditions of temperature, pressure and hydrogen partial-pressure that permit the existence of a minor amount of naphthene in equilibrium with the xylenes and separating the desired xylene isomer from the reaction product. p

2. A process forv recovering a desired xylene isomer, from a mixture of xylene isomers having substantially. less than the equilibrium content of the desired isomer which comprises contacting said xylene mixture in vapor phase and hydrogen'with a hydrogenation-dehydrogenation catalyst under hydrogenating conditions to convert. a substantial proportion of the xylenes to naphthenes and then contacting the hydrogenation reaction product with a hydrogenation-dehydrogenation catalyst under dehydrogenating conditions to convert a substantial proportion of the naphthenes to xylenes and separating the desired" xylene isomer from the reaction product;- 3. A process for recovering a desired xylene isomer: from a mixture of xylene isomers having substantially less than the equilibrium content of the desired isomer which comprises passing said xylene mixture in vapor phase and hydrogen through a bed of a hydrogenationdehydrogenation catalyst, maintaining a partial pressure of hydrogen in the range from 20 to 100 atmospheres in the catalyst bed, maintaining a temperature gradient in the catalyst bed ranging from a temperature of 600 to 800 F. in the portion of the bed adjacent the feed inlet "to'a substantially higher temperature in the range 800 to 1000" F. in the portion of the catalyst bed adjacent the product outlet and separating the desired xylene isomer: from the efiiuent from the catalyst bed.

4. A process for recovering a desired xylene isomer from a mixture of xylene isomers having substantially less than the equilibrium content of the desired isor'neri which comprises contacting said xylene mixture in vapor phase and hydrogen with a hydrogenation-dehydrogenation catalyst at a temperature in the range 700 to 900 F. under a partial pressure of hydrogen ranging from 12 to 18 atmospheres at 700'F. to to atmospheres at 900 F. at a space velocityin the range 0.1 to 10, whereby a substantial proportion of the xylene is hydrogenated to naphthenes and a substantial proportion of the remaining xylenes is isomerized and separating the desired xylene isomer from the reaction product.

5. A process for recovering a desired xylene isomertrom a mixture of xylene isomers having substantially less than the equilibrium content of the desired isomer: which comprises contacting said xylene mixture in vapor f phase and hydrogen with a hydrogenation-dehydrogenation catalyst at a temperature in the range about 700 to 900 F. and under a partial pressure of hydrogen ranging from 12 to 18 atmospheres at 700 F. to 60 to i001 atmospheres at 900 F. at a space velocity in the -};range 0.1 to 10 v./v./hr. whereby a dynamic equilibrium through furnace'8 where it is heated to 950 to 1050" F f if mixture of xylenes and naphthenes having a naphthenic The effluent from e ies its ilar iit sats riise sst s h repeal?! 1 0% by plume was and the individual xylene isomer distribution in the xylene portion of the mixture is substantially an equilibrium distribution, and separating the desired xylene isomer from the reaction product.

6. A process for the isomerization of xylenes which comprises contacting a mixture of xylenes in which the proportion of para-xylene is less than the equilibrium proportion with a clay type cracking catalyst containing about 0.3% platinum at a temperature of about 870 F. and under a pressure of about 33 atmospheres in the presence of about 3 mols of hydrogen per mol of xylene to isomerize the xylenes.

7. A process for the isomerization of xylenes which comprises contacting a mixture of xylenes in which the proportion of para-Xylene is less than the equilibrium proportion with a hydrogenation-dehydrogenation cataiyst at a temperature in the range about 700-900 F. and under a pressure in the range about 1296 atmospheres in the presence of about 3-25 moles of hydrogen per mole of xylene to isomerize the xylenes, the said conditions of temperature, pressure and the amount of hydrogen being correlated such that the product con tains between 10% and 30% naphthenes.

8. A process for the isomerization of xylenes which comprises contacting a feed consisting essentially of a mixture of Xylenes in which the proportion of para- Xylene is less than the equilibrium proportion with a clay type cracking catalyst containing from about 0.3% to about 0.65% platinum at a temperature between about 700 F. and about 900 F. and under a pressure between about 12 atmospheres and about 75 atmospheres in the presence of from about 3 mols to about 6 mols of hydrogen per mol of feed to isomerize the xylenes, the said conditions of temperature, pressure and the amount of hydrogen being correlated such that the product contains between 10% and 30% naphthenes.

References Cited in the file of this patent UNITED STATES PATENTS 1,365,849 Ramage Jan. 18, 1921 2,106,735 Gwynn Feb. 1, 1938 2,328,828 Marschner Sept. 7, 1943 2,470,712 Montgomery et al. May 17, 1949 

1. A PROCESS FOR RECOVERING A DESIRED XYLENE ISOMER FROM A MIXTURE OF XYLENE ISOMERS HAVING SUBSTANTIALLY LESS THAN THE EQUILIBRIUM CONTENT OF THE DESIRED ISOMER WHICH COMPRISES CONTACTING SAID XYLENE MIXTURE IN VAPOR PHASE AND HYDROGEN WITH A HYDROGENATION-DEHYDROGENATION CATALYST UNDER CONDITIONS OF TEMPERATURE, PRESSURE AND HYDROGEN PARTIAL-PRESSURE THAT PERMIT THE EXISTENCE OF A MINOR AMOUNT OF NAPHTHENE IN EQUILIBRIUM WITH THE XYLENES AND SEPARATING THE DESIRED XYLENE ISOMER FROM THE REACTION PRODUCT. 